Tramadol is a well established pain killer, invented by Gruenenthal GmbH, Germany, used as a non-addictive analgesic and sold under different trade names such as Tramal, Ultram, Crispin and Tramundin.
The synthesis of Tramadol is described in U.S. Pat. No. 3,652,589 and in British Patent No. 992,399. The synthesis of Tramadol consists of a Grignard reaction between 2-dimethylaminomethylcyclohexanone and 3-methoxyphenyl magnesium bromide (Equation 1). From the reaction scheme, it is clear that both isomers (RR,SS) (Structure 1) and (RS,SR) (Structure 2) are obtained in variable ratios, depending on the reaction conditions. ##STR1##
The original patents assigned to Gruenenthal GmbH describe the isolation of the (RR,SS) isomer, as follows:
The complex mixture of products containing both isomers of Tramadol obtained from the Grignard reaction is distilled under reduced pressure. The isomers are distilled together at 138.degree.-140.degree. C. (0.6 mm Hg). The distillate is dissolved in ether and is reacted with gaseous HCl. The resulting mixture of both isomers of Tramadol is precipitated as hydrochlorides and filtered. The resulting mixture contains about 20% of the (RS,SR) isomer. The isomer mixture is then refluxed twice with five volumes of moist dioxane, and filtered. The cake obtained consists of pure (RR,SS) isomer. The residual solution consists of "a mixture of about 20-30% of the cis (i.e. RS,SR), which cannot be further separated by boiling dioxane" U.S. Pat. No. 3,652,589, Example 2!.
Dioxane, used in large quantities in this process, possesses many undesirable properties. It has recently been listed as a Category I carcinogen by OSHA Kirk & Othmer, 3rd Ed., Vol. 9, p. 386!, and it is known to cause CNS depression and liver necrosis ibid., Vol. 13, p. 267!; in addition, it tends to form hazardous peroxides ibid., Vol 17, p. 48!. As a result, the concentration of dioxane in the final product has been strictly limited to several ppb's, and the DAC (1991) restricted the level of dioxane in Tramadol to 0.5 ppm.
A different separation method, described in Israeli Specification No. 103096, takes advantage of the fact that the precipitation of the (RR,SS) isomer of Tramadol from its solution in medium chained alcohols (C.sub.4 -C.sub.8) occurs faster than the precipitation of the (RS,SR) isomer, which tends to separate later. The main disadvantage of this method is, that the time interval between the end of separation of the (RR,SS) isomer and the beginning of the (RS,SR) isomer separation is variable, and seems to depend sharply on the composition of the crude mixture. Therefore, variations in the yield and the quality of the product often occur. Furthermore, about 40% of the (RR,SS) isomer does not separate and remains in solution, along with the (RS,SR) isomer. This remaining mixture cannot be further purified by this method.
Another method, described in Israeli Specification No. 116281, relies on the fact that the (RS,SR) isomer of Tramadol undergoes dehydration much faster then the (RR,SS) isomer, when treated with 4-toluenesulfonic acid, or sulfuric acid; furthermore, when the reaction is carried out in an aqueous medium, a certain amount (up to 50%) of the (RS,SR) isomer is converted to the (RR,SS) isomer. This may, of course increase the efficiency of the process.
The unreacted (RR,SS) isomer is then separated from the dehydrated products and from other impurities by simple crystallization.
While further examining the results of the latter process, it was surprisingly found that the hydroxyl group of the (RS,SR) isomer of Tramadol reacts faster than the same group of the (RR,SS) with various reagents. A plausible explanation for this observation can be supplied by comparing the structures of both isomers, and their ability to form hydrogen bonds.
Looking closely at FIG. 1 (RR,SS) Tramadol hydrochloride! and at FIG. 2 (RS,SR) Tramadol hydrochloride!, one can provide a plausible explanation for the difference in the OH group's activity, as follows: The proton attached to the nitrogen atom of the protonated (RR,SS) isomer of Tramadol is capable of forming a stable hydrogen bonding with the oxygen atom of the hydroxyl group (see FIG. 1). Thus, any reaction involving protonation of the hydroxyl group (such as dehydration), or any reaction in which the hydroxyl group reacts as a nucleophile (such as a nucleophilic substitution or esterification process) is less favored to occur.
In the (RS,SR) isomer, on the other hand, there is no possible way of forming a stable intramolecular hydrogen bond, and therefore, any of the above-mentioned types of reactions can easily occur, considering the fact that this particular hydroxyl group is tertiary and benzyllic. ##STR2##
The general purification procedure of the present invention consists of reacting a mixture of both geometrical isomers of Tramadol hydrochloride with a potential electrophile under such conditions that the (RS,SR) isomer reacts almost exclusively, while the (RR,SS) isomer remains practically intact. The resulting mixture is evaporated and the resulting solid substance is then recrystallized from isopropanol or any other suitable solvent.